Solving this puzzle, GCHQ says, “will create an image that leads to a series of increasingly complex challenges.”

At the end of those challenges, it seems there's an email address one can use to notify GCHQ of your answer to the meta-puzzle. The agency suggests that anyone who enjoys the puzzle could do worse than to make a donation to the National Society for the Prevention of Cruelty to Children, a suggestion that works in any season. Whether revealing your cryptographic cleverness to GCHQ makes sense, given its ever-widening surveillance powers, is a question we can't answer charitably. ®